1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer that develops an electrostatic latent image formed on an electrostatic latent image carrier to obtain a visible image. More particularly, it relates to an image forming apparatus having one-component developing device that develops an electrostatic latent image employing a one-component developing agent as a developing agent.
2. Description of the Related Arts
Conventionally, an image forming apparatus like a copying machine or printer having a one-component developing device that develops an electrostatic latent image employing a one-component developing agent has been known. Normally, the one-component developing device is provided with a developing agent carrier such as a developing roller, developing sleeve or a developing belt opposite the electrostatic latent image carrier. The developing agent carrier holds the one-component developing agent on its surface and carries out the developing process by transferring developing agent from a developing agent storage portion to the opposing portion of the electrostatic latent image carrier (referred to "developing region" hereinafter). After developing, residual toner attached to the developing sleeve is returned to the developing agent storage portion by the rotation of the developing sleeve.
The toner returned to the storage portion is removed from the developing sleeve although, one portion of highly charged toner remains on the surface of the developing sleeve. This remaining toner forms a micro-electric field between the itself and the developing sleeve. This micro-electric field draws subsequently supplied toner to the top of the developing sleeve.
However, when a low humidity environment causes the toner to more flow, the charge amount of the toner also increase thus the toner is not removed from the developing sleeve with a tendency for the highly charged toner accumulating on the developing sleeve to increase. Therefore, the amount of toner attracted to the developing sleeve increases and the amount of toner adhering to the sleeve becoming impossible to restrict even by the restricting blade resulting in an abnormal amount of toner being transferred to the electrostatic latent image carrier in the developing region. If an abnormal amount of toner is transferred to the electrostatic latent image carrier in the developing region, problems such as the non-image portion being developed, memory phenomenon in which toner remaining on the developing sleeve develops again, and toner scattering around the periphery of the sleeve will occur.
Furthermore, this results in the toner accumulating on the developing sleeve repeatedly receiving stress from the restricting blade thus adhering to the developing sleeve. This type of phenomenon is called "filming". Filming is the cause of degrading image quality.
Further, when the toner on the developing sleeve receives stress, toner become to have a small diameter or fluid silica contained in the toner is peeled. As a result, reproducibility of black portions of the image is poor.
Even further, when the toner accumulating on the developing sleeve increases, toner newly supplied to the developing sleeve is not only charged by the restricting blade but is also charged by the friction of the other toner. Therefore, the charging polarity of the toner opposite that of the regular charging polarity increases resulting in deterioration of image quality.
In one such disclosure of a developing device in U.S. Pat. No. 4,930,438, a charge erasing brush is provided to weaken the electrostatic adhesion force of the residual toner on the developing sleeve. A power supply with a voltage identical to the developing bias voltage applied to the developing sleeve applies a charge erasing bias voltage to this charge erasing brush. The charge erasing brush then erases unnecessary accumulating charge from the residual toner using this applied voltage to weaken the electrostatic adhesion force of the residual toner on the developing sleeve.
However, when a voltage identical to the developing bias voltage is applied to the charge erasing brush, it becomes impossible to erase the charge of a voltage lower than the developing bias voltage even if an accumulated charge higher than the developing bias voltage can be removed.